Rail bosses have blamed fire safety and anti-vandalism rules after passengers complained new uncomfortable seats feel like "solid metal".
Govia Thameslink admits the seats on the new Class 700 trains are considerably firmer than older designs. The seats have limited padding, because cushioning is deemed as a fire hazard by the Department for Transport (DfT), according to the rail operator.
They also have to be made from graffiti-proof material, which the DfT said has a "firm feel".
Travellers have complained the seats are hard and narrow, with some joking that they will be bringing cushions to sit on in future.
Frustrated passengers have tweeted about their uncomfortable journeys, with one complaining: "the brand new Thameslink trains have ironing boards for seats."
Another said the seats are "hard, near vertical and narrow," and one passenger advised bringing "a small inflatable cushion if you’re unlucky enough to get a seat."
The "Tube-style" trains were developed to ease overcrowding at rush hour through London, and have fewer seats in each carriage than previous rolling stock, to make room for more standing space.
However, unlike the London Underground, the trains are designed for those who routinely take journeys which span over an hour.
A spokesperson for Govia Thameslink told The Sunday Telegraph: "The Class 700 trains are essential to the transformation we are making to meet passengers' needs for dramatically greater capacity and frequency to and through central London.
"Various seat designs were tested with passengers. The chosen DfT-approved design is the one that met all the safety and capacity requirements and offered the best comfort.
Thameslink, travel in style and comfort with our "looks like a cushion but is actually solid metal" seats.— TheresNoTime (@TheresNoTimeFor) February 6, 2018
"Padding in the seats is constrained by the latest fire standards, which are very strict on modern trains, and is also vandal resistant; this does give it a firmer feel. The shape and size of the seats had to meet today's extremely strict crash-worthiness standards."
The roll out began in 2016, and by the end of 2018, there will be 115 of the German-built trains Govia’s Thameslink and Northern services from London to Cambridge and Peterborough, as well as Brighton and Bedford.